1. Field of the Invention
The invention relates to a fuel injector with direct needle control for an internal combustion engine.
2. Description of the Prior Art
Fuel injectors with a so-called direct needle control are known. Fuel injectors of this kind function without a control valve interposed between an electrically controlled actuator and a nozzle needle. The transmission of force between the actuator and the nozzle needle is implemented by means of a pressure boosting unit. Suitable actuators for this are in particular piezoelectric actuators, which have a direct or inverse triggering depending on whether or not they are supplied with current in the closed state. With a direct triggering, the piezoelectric actuator is supplied with current in order to open the nozzle needle so that a linear expansion of the piezoelectric actuator, through a pushing motion, causes an opening of the injection nozzles, the motion being amplified by the pressure boosting unit. In the closed state, the piezoelectric actuator therefore has a shorter longitudinal span. With an inverse triggering, the piezoelectric actuator is supplied with current in the closed state of the nozzle needle so that when the piezoelectric actuator is in the elongated state, it holds the nozzle needle closed. When the piezoelectric actuator is triggered to initiate the injection process, it is switched into a currentless state so that a pulling motion of the piezoelectric actuator causes a pressure drop in a control chamber of the pressure boosting unit. This hydraulically boosts the stroke motion of the piezoelectric actuator for opening the nozzle needle.
In fuel injectors with direct needle control, in order to be able to open the injection nozzles directly by means of the actuator, the actuator must overcome a powerful closing force. The opening force that the actuator must exert results from the fact that the nozzle needle is pressed into its seat by system pressure (the pressure level in the high-pressure accumulator). Lifting the nozzle needle away from its seat can require forces of up to 400 Newton. In order to assure a sufficient fuel flow when the injection nozzles are completely open during an injection into the combustion chamber of an autoignition internal combustion engine, it is also necessary for the nozzle needle to execute a maximum stroke of several 100 μm. Although the integration of a hydraulic booster unit does permit one to vary the length-to-diameter ratio of the piezoelectric actuator, the size of the actuator—also referred to as actuator volume—remains essentially proportional to the opening force to be exerted and to the maximum nozzle needle stroke distance to be achieved.
DE 10326046 A1 has disclosed various embodiments of fuel injectors with direct needle control. To this end, the fuel injector has a nozzle needle, which is guided in a nozzle body and acts on a nozzle needle sealing seat, and also has a piezoelectric actuator and a hydraulic pressure boosting unit. The pressure boosting unit has a hydraulic coupler and/or control chamber operatively connected to an actuator pressure booster piston, which is connected to the actuator, and is also operatively connected to a nozzle needle pressure booster piston, which is connected to the nozzle needle.